Leak-down test...Analysis?

Originally Posted by Paul Workman

At idle the pressure was steady at 43#. I shut off the ignition, and after 20 minutes, it had leaked down by only 2#. I concluded the injectors are at least not leaking.

Paul, don't worry about a leak down over a 20 minute period. What's the leak down after 30 seconds?

Also injectors can fail on several issues. One being stuck open or closed, but also in the nozzle with puddling or not atomizing correctly.

[Paul Workman]

Originally Posted by Jeffvette

Paul, don't worry about a leak down over a 20 minute period. What's the leak down after 30 seconds?

Also injectors can fail on several issues. One being stuck open or closed, but also in the nozzle with puddling or not atomizing correctly.

Thanks for that info, Jeff. After 30 seconds, there was no appreciable drop in pressure that I noticed. Yes, I understand about "other issues" injectors might have; e.g. sticking, puddling, etc. I was doing the test looking for an "ah-HA!" kind of reason to go plenum diving. I'll check for vacume "leaks" and run a dynamic scan when it is acting up and see what that turns up before diving in. (I prefer to troubleshoot by isolating the problem, as you do, rather than "shotgunning" i.e. changing parts until the problem goes away as they love to do at most repair shops.) So, any other clues you might have, and I'm all ears.

I might mention too, that after it has been redlined a couple times on an outing, the problems seem to go away. The stumbiling only returned after 5-6 periods of idling for about 20 minutes at a time until normal operating temp was reached. I guess that would be 6x20 or about 2 hours of idling and most of that time the temp was significantly below operating temp...Methinks that is a pretty tall order for any vehicle, now that I think about it...[sigh].

I've about concluded that injectors might be necessary, but I think I'll change the routine and avoid idling - waiting for those occational dry days for a few winter joyrides and see what effect that has on the stumbling characteristic.

My only point of reference was my previous '95 LT1, and it didn't seem to get "loaded up" as result of idling in the same fasion. But, this LT5 is new to me, and so I'm not sure what is or is not normal. I just may have to pull the plenum and "do" the injectors and see what happens (hence my quest for info on LT5 characteristics...I'm in "learn" mode in that regard!).

Jeff, your comments (and those of the others too) are always welcome - and I'm grateful to have 'em. I certainly would like to fast-track my schooling on this classic Corvette Z as is possible! Now that I have the FSM (thanks, Santa), I kin get me some book learnin' an be paper shmardt at least! Now I can really get into trouble!!

P.

[HIZNHRZ]

Haven't done a leakdown test on an LT5 but I have wrestled with a rough idle problem. I agree that eliminating prossible causes one at a time is the way to go. Jeff probably knows better than anyone but it seems to me that a likely root cause for rough idle once injectors are ruled out are vacuum leaks. My injectors turned out to be a significant cause of my problems but under-the-plenum (poor) seals on some very old vacuum lines was also a major issue.

[Aurora40]

One quick way to tell if air leaking in is a problem is to hook up a scan tool/laptop and look at the IAC counts at idle. If there are counts, then the ECM is in control of the idle. If there are zero counts, you have a reasonable leak somewhere (especially if the idle is above the desired idle rpm).

Paul, let's start with something simple. Change the plugs. Try the NGK 6341 plugs.

[Paul Workman]

Originally Posted by Jeffvette

Paul, let's start with something simple. Change the plugs. Try the NGK 6341 plugs.

Check! I've got em coming and will be in hand tomorrow morning.

While I'm at it, I'll try the propane trick and check for air leaks as well - at least in the accessible parts of the plenum and hose connections. Stay tuned! (Ahl B back!!)

P.

Paul, I think what you will find is the plugs will clean up your miss. It doesn't sound like you are having any other issues. I'm guessing your plugs are probably originals.

[Tyler Townsley]

Also it can be old/bad gas. Run at least 1 tankful using a good injector cleaner after changing plugs.

Tyler

[bradslt5]

what is the best inj cleaner to use?

[tomtom72]

I was told the good stuff is Techron by Chevron. Black bottle any autozone or Costco has a 4 pack for about $20.00.

Paul, not to be forward.....I notice with mine that around town driving is the worst on my car. I go out with my vette buddies & try to keep up and the car never runs better...for a few days untill my around town driving "loads" it up again...if I drive it like a wuss = car don't seem to like it. If I beat it with an ugly stick = car seems to laught at me & wants more & she runs sweet. Oh, I'm using the std GM replacement plug for the FR2LS's that were OEM in my 90.


Tom